1. Field of the Invention
The present invention relates to a liquid phase growth method for producing various semiconductor crystals and optical crystals adapted for use in semiconductor devices and electro-optical devices, and a liquid phase growth apparatus suitable for carrying out the method.
2. Related Background Art
Solar cells have become adopted widely for consumer use with the recently increased concern for the environment. Monocrystalline or polycrystalline silicon is principally used as the semiconductor material for the solar cells for consumer use.
At present, such crystals are cut out from a large ingot by slicing in the form of a wafer of a thickness of about 300 xcexcm. Such a method is insufficient in the utilization efficiency of the material, since a slicing margin of about 200 xcexcm will be required in the slicing operation. In order to attain a larger production amount and a lower cost hereafter, it is desired to grow and use a crystal of a minimum thickness required electrically or optically of several 10 to about 100 xcexcm. For growing a crystalline silicon of such a small thickness, there has principally been investigated the gaseous phase growth method in which a silicon-containing gas is decomposed by the action of heat or plasma.
However, in the mass production of solar cells, there is required an apparatus capable of growing silicon at a rate of 1 xcexcm/min or more on several tens to several hundreds of square substrates of 4 to 5 inches square in a single batch. The gaseous phase growth apparatus that meets such requirement is not commercially available.
For crystal growth, there is also conventionally known the liquid phase growth method, and the method is practically employed in producing compound semiconductor crystals for LEDs and optical crystals for electro-optical elements. Recently, as disclosed in Japanese Patent Application Laid-Open No. 10-189924, there has been reported utilization of a crystalline silicon film grown on a crystalline silicon substrate or a ceramic substrate for the production of the solar cells.
In the liquid phase growth method, a melt is prepared firstly by heating to fuse a metal such as tin, indium or gallium or an oxide such as a lithium oxide or niobium oxide. Then, a material for constituting a crystal such as arsine or silicon is melted in the melt as needed, then a substrate is dipped in the melt and the melt is brought into supersaturation by, for example, cooling to deposit a crystal on the substrate. The liquid phase growth method is not only capable of growing a crystal of good quality but also is less in the waste of a material that does not contribute to the crystal growth as compared with the gaseous phase growth method and is therefore suitable for application to a device such as a solar cell for which a low production cost is strongly required or an electro-optical device using an expensive material such as gallium or niobium.
However, since the liquid phase growth method has been limited in its application, the apparatuses that have been commercially available are limited to those for growing a compound semiconductor on a substrate of 3 inches or less in diameter and have had few applications for silicon growth.
In consideration of the foregoing, the conventional liquid phase growth apparatuses have been studied and found to have the following problems.
FIG. 14 is a schematic view showing an example of the conventional liquid phase growth apparatus capable of crystal growth on a plurality of substrates. In this apparatus, five substrates 201 are horizontally supported at predetermined distances by substrate support means 202, and are immersed in a melt 204 held in a crucible 203 of a cylindrical shape with a bottom, and these components are housed in a growth chamber 205. The temperature of the melt 204 can be suitably controlled by an electric furnace 206. The growth chamber is provided with and can be suitably opened or closed by a gate valve 207.
In this apparatus, firstly, substrates 201xe2x80x2 for melting in a melt (represented as xe2x80x9c201xe2x80x2xe2x80x9d with a prime in order to distinguish them from substrate for crystal growth and hereinafter sometimes referred to simply as xe2x80x9cmelting substratesxe2x80x9d) made of a crystal raw material (i.e., material to be grown) such as silicon are supported by the substrate support means 202, and are immersed in a melt of a low-melting metal such as tin, indium or gallium or an oxide such as a lithium oxide or niobium oxide, as heated to a predetermined temperature by the electric furnace 206, to melt the crystal raw material to the saturation state at the temperature, thereby preparing the melt 204. Thereafter, the melting substrates 201xe2x80x2 are lifted up from the melt 204 and are replaced by the substrates 201 for crystal growth (hereinafter sometimes referred to simply as xe2x80x9cgrowth substratesxe2x80x9d). (Therefore, the growth substrate 201 and the melting substrate 201xe2x80x2 cannot be distinguished from each other in the figures.)
Thereafter, when the melt 204 is gradually cooled to a predetermined temperature and the growth substrates 201 are immersed therein, the raw material which is no longer soluble in the melt 204 starts to deposit on the surfaces of the substrates 201, so that crystals such as of silicon will grow on the substrates. At this time, a polycrystalline film is grown when the substrate 201 is made of polycrystalline silicon, glass or ceramic, but a monocrystalline film can be grown when the substrate 201 is made of monocrystalline silicon. The substrates 201 are lifted up when the crystals are grown to a predetermined thickness.
By carrying out the mounting or detaching of the substrates 201 to or from the substrate support means 202 with the gate valve 207 being closed, and by changing the interior of the load lock chamber 208 from the atmosphere to an inert gas or the like and thereafter opening the gate valve 207 and lowering the substrates 201 into the growth chamber 205, the melt 204 can be prevented from reacting with oxygen or water or from being contaminated.
In the apparatus shown in FIG. 14, the number of substrates can be increased as needed. However, it has been experimentally found that the above-described configuration is insufficient for obtaining a high growth rate uniformly over the plane.
FIG. 15 is a chart showing the in-plane distribution of the growth rate in a case where five silicon wafers of a diameter of 5 inches, maintained at a mutual distance of 1 cm, are subjected to silicon crystal growth from an indium melt. In the figure, the symbol xe2x80x9c∘xe2x80x9d indicates the distribution in the substrates positioned close to the surface layer portion of the melt, while xe2x80x9cxe2x80xa2xe2x80x9d indicates that in the substrates positioned close to the bottom of the melt.
Although the difference between the substrates is small, the growth rate at the central portion of each substrate is about ⅓ of that at the peripheral portion, as shown in FIG. 15. Further, decreasing the cooling rate of the melt reduces the ununiformity within the plane but also decreases the entire growth rate. Further, increasing the distance between the substrates reduces the ununiformity but decreases the number of substrates that can be charged in a single batch. Thus, the both means will result in lowering in the throughput.
The ununiformity of the growth rate within the plane results from the fact that a fresh melt cannot be supplemented in a sufficient amount after the crystallization of the semiconductor raw material melted in the melt present between the substrates, and the ununiformity becomes more remarkable as the deposition rate becomes large or the distance between the substrates is made smaller. In the apparatus shown in FIG. 14, by rotating the substrates during the crystal growth, the uniformity of the growth rate is somewhat improved since a melt containing silicon at a high concentration is supplemented between the substrates, but is still insufficient.
For causing mutual movement of the melt and the substrates, there can also be conceived a method of rotating the crucible while maintaining the substrates stationary. Rotation of the crucible kept at a high temperature is already adopted generally in the single crystal pulling up apparatus of the Czochralski method. Japanese Patent Application Laid-Open No. 7-315983 proposes a configuration in which rotation of a crucible is applied to a liquid phase growth apparatus.
However, in a method of merely rotating a crucible, the melt does not sufficiently follow the rotation of the crucible, so that the melt agitating effect is small although the apparatus becomes complicated in its structure.
Further, Japanese Patent Application Laid-Open No. 5-330979 discloses a method of liquid phase growth by arranging substrates vertically and dipping them in a melt. The configuration of this liquid phase growth apparatus is shown in FIG. 16, wherein reference numeral 310 denotes an electric furnace, 303 a crucible, 301 a solution, 312 growth substrates mountable on a supporting rod, 315 a supporting rod, 317 a drive means, 318 a support stand, and 319 a lifter.
As shown in FIG. 16, the growth substrates 312 are supported perpendicularly to the surface of the solution 301, and are dipped in the solution 301 by the lowering of the supporting rod 315 whereby the crystal growth is started. The substrates 312 are rotated with the supporting rod 315 being the center.
The rotation of the substrates in the flat planes thereof reduces the variation of the film thickness among the substrates, but the substrate has to have an opening for mounting on the supporting rod, which portion cannot be used. Further, since the substrate is rotated at a constant location with the center thereof being an axis, the supply of the solution to the substrate surface is limited and the film formation rate cannot be made larger. Furthermore, it is difficult for the conventional arrangement of the substrates to process a number of substrates in a single step.
Further, Japanese Patent Application Laid-Open No. 6-206792 proposes providing baffle plates in a vertical direction on the inner wall of a crucible. However, the proposal has a limited effect in moving the melt from below to above since the baffle plates are provided only on the sidewall. Furthermore, the flow of the solution during the crystal growth is small since the substrates are held horizontally.
Further, Japanese Patent Application Laid-Open No. 7-277876 proposes providing fins on the bottom of a crucible and rotating the crucible, but the flow of a solution in the crucible is insufficient because the fins are provided only at the bottom portion.
Further, Japanese Patent Application Laid-Open Nos. 55-140793 and 2001-39791 propose providing fins at the bottom of a crucible independently from the crucible and rotating the fins to cause a flow of a solution, but the provision of the rotating fins independently from the crucible complicates the structure of the apparatus to increase the production cost thereof. Further, the flow of the solution is insufficient because no vertical fins are present in the vicinity of the wall surface.
The present invention has been accomplished in consideration of the above-mentioned problems, and an object of the present invention is to provide a liquid phase growth method capable of providing a high growth rate and showing little difference in the growth rate among the substrates or within the same substrate even when a number of substrates are charged in one batch, and an apparatus suitable for carrying out the method.
The above-mentioned object can be attained, according to a first aspect of the present invention, by a liquid phase growth method comprising dipping a seed substrate in a solution (i.e., molten metal) in a vessel having a crystal raw material melted therein and growing a crystal on the substrate, wherein a fin is provided on a bottom of the vessel, for regulating (or directing) a flow of the solution from a central portion outside in a radial direction in the vessel; a flow-regulating plate is provided in the vicinity of an inner sidewall of the vessel, for regulating a flow of the solution from the bottom upwardly; and the vessel is rotated while regulating a flow of the solution by an action of the fin and the flow-regulating plate to bring the solution into contact with the seed substrate.
In the first aspect of the present invention, it is preferred to hold the substrates perpendicularly to a surface of the solution and to arrange the substrates, as viewed from above, radially from a central portion outside in a radial direction in the vessel and equidistantly in a circumferential direction of the vessel.
Further, it is preferred to encircle the substrates with a cylindrical member and to provide a plurality of the flow-regulating plates on an outer peripheral surface of the cylindrical member.
Moreover, it is preferred to provide a cylindrical member at a central portion of the vessel.
Further, it is preferred to rotate the vessel in a circumferential direction thereof, alternately in forward and backward directions.
According to a second aspect of the present invention, there is provided a liquid phase growth apparatus comprising a vessel for housing a solution having a crystal raw material melted therein and substrate support means for supporting and dipping a seed substrate in the solution, wherein a fin is provided on a bottom of the vessel radially in a radial direction thereof; a flow-regulating plate is provided in the vicinity of an inner sidewall of the vessel perpendicularly to a surface of the solution; and the vessel is provided with rotating means.
In the second aspect of the present invention, it is preferred that a plurality of the substrates are supported by the substrate support means perpendicularly to the surface of the solution, and the substrates, as viewed from above, are arranged radially from a central portion outside in a radial direction in the vessel and equidistantly in a circumferential direction of the vessel.
Further, it is preferred that a cylindrical member is provided in the vessel so as to encircle the substrates and the flow-regulating plate is provided vertically on an outer peripheral surface of the cylindrical member.
Moreover, it is preferred that a cylindrical member is provided at a central portion of the vessel such that the substrates are positioned surrounding the member.
Further, it is preferred that an angle formed by a side face of the fin provided on the bottom of the vessel and the bottom face of the vessel becomes a larger obtuse angle toward the outside in the radial direction.
That is, in the first and the second aspects of the present invention, in consideration of the fact that the difference in the crystal growth rate within the same substrate or among substrates results from the decrease of the crystal raw material melted in the solution in the vicinity of the substrate during the growth of the material on the substrate, by sufficiently flowing the solution in the crucible during the growth of the material on the substrate, the decrease of the crystal raw material in the solution in the vicinity of the substrate is effectively prevented.
According to a third aspect of the present invention, there is provided a liquid phase growth method comprising dipping a substrate in a solution housed in a vessel and containing a crystal raw material, and liquid-phase growing the material on the substrate while moving the substrate in a plane direction thereof in the dipped state to agitate the solution.
In the third aspect of the present invention, it is preferred to move the vessel in a direction opposite to the moving direction of the substrate.
Further, it is preferred to arrange the substrates radially and to rotate the substrates to agitate the solution.
According to a fourth aspect of the present invention, there is provided a liquid phase growth apparatus comprising a vessel housing a solution containing a crystal raw material, dipping means for dipping a substrate in the solution, and moving means for moving the substrate in a plane direction thereof in a state in which the substrate is dipped in the solution by the dipping means.
In the fourth aspect of the present invention, it is preferred to further provide means for moving the vessel in a direction opposite to the moving direction of the substrate.
Moreover, it is preferred to further provide substrate support means for supporting the substrates radially and to rotate the substrate support means to agitate the solution.
Further, it is preferred that a rib is provided on an inner face of the vessel and the thickness of the rib decreases in a direction in which the solution as solidified is taken out.
Moreover, it is preferred that ribs are provided on the side face and the bottom face of the vessel.
That is, in the third and the fourth aspects of the present invention, in consideration of the fact that the difference in the crystal growth rate within the same substrate or among substrates results from the decrease of the crystal raw material melted in the solution in the vicinity of the substrate during the growth of the material on the substrate, by sufficiently agitating the solution during the growth of the material on the substrate, the decrease of the crystal raw material in the solution in the vicinity of the substrate is effectively prevented.
According to a fifth aspect of the present invention, there is provided a liquid phase growth method comprising dipping a seed substrate in a solution in a vessel having a crystal raw material melted therein and growing a crystal on the substrate, wherein a first fin is provided on a bottom of the vessel, for regulating a flow of the solution from a central portion outside in a radial direction in the vessel; a second fin is provided on an inner sidewall of the vessel, for regulating a flow of the solution from the bottom upwardly; and the vessel is rotated while regulating a flow of the solution by an action of the first and the second fins to bring the solution into contact with the seed substrate.
In the fifth aspect of the present invention, it is preferred that the crystal is grown on the substrate while moving the substrate in a plane direction thereof to agitate the solution.
Further, it is preferred that the vessel is moved in a direction opposite to the moving direction of the substrate.
Moreover, it is preferred that the substrates are arranged radially and are rotated to agitate the solution.
According to a sixth aspect of the present invention, there is provided a liquid phase growth apparatus comprising a vessel for housing a solution having a crystal raw material melted therein and substrate support means for supporting and dipping a seed substrate in the solution, wherein a first fin is provided on a bottom of the vessel radially in a radial direction thereof and a second fin is provided on an inner sidewall of the vessel.
In the sixth aspect of the present invention, it is preferred that the vessel is provided with rotating means.
Further, it is preferred that the substrate support means is for supporting the substrates radially and is provided with rotating means.
Moreover, it is preferred that a flow-regulating plate is provided in the vicinity of an inner sidewall of the vessel perpendicularly to a surface of the solution.
According to a seventh aspect of the present invention, there is provided a liquid phase growth apparatus comprising a vessel for housing a solution having a crystal raw material melted therein and substrate support means for supporting and dipping a seed substrate in the solution, wherein a flow-regulating plate is provided in the vicinity of an inner sidewall of the vessel perpendicularly to a surface of the solution.